The Republic of New Atlantis was a micronation created by Leicester Hemingway (1 April 1915, Oak Park, Illinois – 13 September 1982, Miami Beach, Florida), the younger brother and biographer of writer Ernest Hemingway.

As the location of what he would later describe as "the world's smallest political entity", Hemingway chose a site above a small seamount in what were then international waters, 12.87 kilometres southwest of Jamaica.

At the time of its foundation, on 4 July 1964, the territorial pretensions of New Atlantis anticipated the future construction of a large, habitable man-made island; in practice they were limited to a 2.44 x 9.14 metre steel and bamboo raft, which Hemingway anchored to the floor of the Carribean Sea with the aid of an old Ford engine block.

Hemingway proposed to use the novelty of the New Atlantis concept to generate capital for the establishment of an oceanographic research institute; these plans appear to have received the tacit endorsement of the Jamaican government, whose US ambassador is reported in the New York Herald Tribune as describing Hemingway as a “decent, well-meaning soul,” and New Atlantis as variously “good” and “sound.”

The project came to an abrupt end in 1966, when the raft was destroyed during a tropical storm. Some reports suggest that many of the structure's component materials were pillaged by local fishermen prior to its final destruction.